Scientists uncover the source of an almost 2 billion year delay in animal evolution

A deficiency of oxygen and the heavy metal molybdenum in the ancient deep ocean may have delayed the evolution of animal life on Earth for nearly two billion years.

Dr Simon Poulton, Civil Engineering and Geosciences, Newcastle University, was part of an international team of biogeochemists who took part in the University of California-led study.

The study's results are published in today's edition of Nature (27th March).

'For decades it was assumed that the ocean became oxygenated shortly after an initial rise in atmospheric oxygen about 2.4 billion years ago,' said Dr Poulton. 'This study provides independent confirmation that there was a major delay in the oxygenation of the ocean, and furthermore, it now appears that the availability of molybdenum may have played a crucial role in animal evolution.

'At last, a coherent picture of the environmental conditions that led to the evolution of animal life is emerging.'

The researchers arrived at their conclusion after tracking molybdenum in black shales, a kind of sedimentary rock rich in organic matter found in the ocean. Molybdenum is a key micronutrient for the life-forms that control the production of oceanic and atmospheric oxygen.

Following the initial rise of oxygen in the Earth's atmosphere 2.4 billion years ago, oxygen was transferred to the surface ocean to support oxygen-demanding micro-organisms. However, the diversity of these single-celled life forms remained low, and their multi-cellular ancestors (animals) did not appear until about 600 million years ago.

Suspecting that deficiencies in oxygen and molybdenum might explain this evolutionary lag, the team measured the abundance of molybdenum in ancient marine sediments over time to estimate how much of the metal had been dissolved in the seawater in which the sediments formed.

The researchers found significant, firsthand evidence for a molybdenum-depleted ocean compared to the high levels measured in today's oxygen-rich seawater.

'These molybdenum depletions may have retarded the development of complex life such as animals for almost two billion years of Earth's history,' said project leader Professor Timothy Lyons, at the University of California's Department of Earth Sciences. 'The amount of molybdenum in the ocean probably played a major role in the development of early life.

'As in the case of iron today, molybdenum can be thought of as a life-affirming micro-nutrient that regulates the biological cycling of nitrogen in the ocean.

'At the same time, molybdenum's low abundance in the early ocean highlights the global extent of oxygen-poor seawater and implies that the amount of oxygen in the atmosphere was still low.

'Knowing the amount of oxygen in the early ocean is important for many reasons, including a refined understanding of how and when appreciable oxygen first began to accumulate in the atmosphere.

'These steps in oxygenation are what ultimately gave rise to the first animals almost 600 million years ago - just the last tenth or so of Earth's history.'

For animal life to commence, survive and eventually expand on Earth, a threshold amount of oxygen - estimated to be on the order of 1 to 10 percent of present atmospheric levels of oxygen - was needed.

Past research has shown that Earth's oxygenation occurred in two major steps: The first step, around 2.4 billion years ago, took place as the ocean transformed to a state where only the surface ocean was oxygenated by photosynthesizing bacteria, while the deep ocean was relatively oxygen-free.

The second step, around 600 million years ago, marked the point when the entire ocean became fully oxygenated through a process not yet fully understood. The purpose of this research was to find out the state of the ocean between the two steps.

By tracking molybdenum in shales rich in organic matter, researchers found the deep ocean remained oxygen and molybdenum-deficient after the first step. This condition may have had a negative impact on the evolution of early eukaryotes, our single-celled ancestors. The molybdenum record also shows that the deep ocean only became fully oxygenated by around 550 million years ago.

According to this research, the timing of the oxygenation steps suggests that significant events in Earth's history are related. Scientists have long speculated that the evolution of the first animals was somehow linked to the so-called Snowball Earth hypothesis, where the Earth was covered from pole to pole in a thick sheet of ice for millions of years. Oxygenation of the oceans and the evolution of animal life occurred shortly after the last of Earth's global glaciations.

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